Real-world scenes often exhibit a wider range of brightness than can be captured by most camera system with a single exposure level. A captured image, e.g., by a digital camera, may include both very bright regions and very dark regions. Ideally, a photograph of a large brightness range scene would include both the details in the bright regions and the details in the dark regions. However, due to dynamic range limits of software and hardware of the imaging device, e.g., a digital still camera, an image capturing a high contrast scene is usually incapable of preserving the high dynamic range (ratio between dark and bright regions) of the entire scene as its original appearance. Typically, in a single exposure setting, the very bright region tends to become saturated and the very dark region tends to become under-exposed, and thereby can hardly be reproduced in one captured frame without distortion.
One technique used to obtain an image with a high dynamic range is by capturing multiple still images of the same resolution having different exposure levels, and then combining the images into a single output image having increased dynamic range. Another method for obtaining a high dynamic range image is the simultaneous capture of multiple images having different exposure levels. The images are subsequently combined into a single output image having increased dynamic range. This capture process can be achieved through the use of multiple imaging paths and sensors.
A recently developed technology, e.g., the so-called “Always-on High Dynamic Range (AOHDR),” takes a different approach in which the image sensor of an imaging device are programmed to capture a short exposure image and a long exposure image simultaneously in different pixel locations. The short and long exposure images are interleaved in the raw mosaiced output of the sensor, typically in a Bayer array pattern resulted from the use of a Bayer filter on the image sensor. FIG. 1 illustrates a sample Bayer mosaic array representing interleaved high dynamic range (HDR) image data including alternating pairs of scanlines of short exposure and long exposure pixels. The Bayer array basically is composed of repeating units of 2×2 pixels, including one R, one B and 2Gs. In this interleaved pattern, each two adjacent long exposure pixel rows, e.g., 101A and 101B, alternates with two adjacent short exposure pixel rows, e.g., 102A and 102B. The short and long exposure images are captured contemporaneously by the digital camera system.
The process of deinterleaving the interleaved Bayer array can be complicated. Taking FIG. 1 as example, the long exposure and short exposure rows do not have the same pixels, but are spatially aligned with the source sensor cells. Therefore, to deinterleave the image, short and long exposure lines need to be recreated from adjacent pairs of short and long exposure lines. Conventionally, an interleaved raw image is deinterleaved in the Bayer mosaic space. Unfortunately, this has significantly lower spatial resolution than the original image and leads to sampling artifacts. The lower spatial resolution is a result of the fact that one can only perform arithmetic operations between components of the same color filter value, e.g., red with red, green with green, and blue with blue. For example, the attempt to deinterleave the red (R) component has only ¼ of the possible image information available. These sampling artifacts in both luma and chroma, or the aliasing effect, are visually objectionable and yet are common in high-contrast images.